Permanently eccentered formation tester

ABSTRACT

A formation tester for determining the formation pressure of a subsurface formation traversed by a wellbore comprises of:—an elongate tester body;—a support plate ( 3 ) that is extendible outwardly from the surface of the formation tester body, said support plate carrying probe means ( 5 ) to establish a passageway between the inside of said formation tester body and said formation, and a sealing pad ( 6 ) connected to said probe means to isolate said passage way between the inside of said formation tester body and said formation;—anchoring means ( 7 ) to settle said tester body at a level within the wellbore. The elongate tester body comprises an eccentric portion ( 2 ) wherein said support plate is mounted such that a determined standoff is maintained between said elongate tester body and the wall of the wellbore when said tester body is settled at a level in the wellbore.

The present invention relates generally to the field of oil and gasexploration. More particularly, the invention relates to a permanentlyeccentered formation tester for determining at least one property of asubsurface formation penetrated by a wellbore.

Over the past several decades, highly sophisticated techniques have beendeveloped for identifying hydrocarbons, commonly referred to as oil andgas, from subsurface formation. These techniques facilitate thediscovery, assessment and production of hydrocarbons from subsurfaceformations.

When a subsurface formation containing an economically producible amountof hydrocarbons is believed to have been discovered, a borehole istypically drilled from the earth surface to the desired subsurfaceformation and tests are performed on the formation to determine whetherthe formation is likely to produce hydrocarbons of commercial value.Typically, tests performed on subsurface formation involve interrogatingpenetrated formations to determine whether hydrocarbons are actuallypresent and to assess the amount of producible hydrocarbons therein.These preliminary tests are conducted using formation testing tools.These formation testing tools are typically lowered into a wellbore by awireline cable, tubing, drill string or the like and may be used todetermine various formation characteristics which assist in determiningthe quality, quantity and conditions of the hydrocarbons or other fluidslocated therein. Other tools may form part of drilling tool, such asdrill string for the measurement of formation parameters during thedrilling process.

Formation testing tools usually comprise cylindrical bodies adapted tobe lowered into a borehole and positioned at a depth in the boreholeadjacent to the subsurface formation for which data is desired. Oncepositioned in the borehole, these tools are placed in fluidcommunication with the formation to collect data from the formation. Inorder to establish such fluid communication, a probe, snorkel or otherdevice is sealed against the borehole wall.

Formation testing tools, also called formation testers, are used tomeasure downhole parameters such as wellbore pressures, formationpressures, and formation mobilities among others. They may also be usedto collect samples from a formation so that the types of fluid containedin the formation and other fluid properties can be determined. Theformation properties retrieved during a formation test are importantfactors in determining the commercial value of a well and the manner inwhich hydrocarbons may be recovered from it.

However, retrieving such formation properties with a formation testermay cause some problems. The pressure of the wellbore fluid, alsoreferred to as mud, must be maintained at a higher level than thepressure of the formation, to prevent the formation fluid from flowingout of the formation and rising very quickly to the surface. Variouschemical constituents are added to the mud to increase its density andoverall weight, and increase the pressure of the wellbore fluid,referred to as the hydrostatic pressure or mud pressure. The differencebetween the mud pressure and the formation pressure is referred to asthe pressure differential. This difference can be as high as 5000 psi,but is most often 2000 psi or less. If the pressure differential ispositive, then fluid and solid content of the mud will tend to flow intothe formation. If the pressure differential is negative, then fluid andsolid content of the formation will tend to flow from the inside of theformation to into the wellbore and upwards towards the surface. If apositive differential pressure is maintained, then wellbore fluid andsolid particles will flow from the wellbore into the formation, and thesolid particles will stack up against the wall of the wellbore. Overtime, these stacked particles will create a seal between the wellboreand the formation, said seal being referred to as the mudcake. If themudcake is removed from the wall of the wellbore, and if a positivedifferential pressure still exits, then the contents of the wellboreagain will begin to flow into the formation and a new mudcake will beformed. The mudcake can be up to ½ inch or greater in thickness,depending on the permeability of the formation, mud type, drillingoperations and procedures and pressure differential.

If the mudcake is removed or disturbed while a formation tester islowered into the well, then the formation tester can be drawn towardsthe wall of the wellbore due to the differential pressure and becomestuck to said wall. The phenomenon is known as differential sticking.The probability for the tester to be differentially stuck isproportional to four main variables: area of mudcake that has beenremoved or disturbed, amount of positive differential pressure, surfacearea of the tester that is in contact with the area of removed mudcakeand time the formation tester surface area is in contact with area ofremoved mudcake.

Formation testers known in the state of the art have a significant riskof becoming differentially stuck. This risk can mainly be attributableto the large size and length of formation testers and the tendency ofthis tool to remove the mudcake while being lowered into the well. Thisrisk is also due to poor positioning of the formation testers in thewellbore, such that large surface of the tool can be in contact with thearea of removed mudcake. This poor positioning is due to usual tooldesign that comprises, on one side of the tool, an anchor to set thetool in place at a certain level in the well and opposite to the anchor,a probe that will perform the measurements. The probe and anchor forcesare traditionally identical and exactly opposing. Furthermore, the probeand anchor are able to extend independently of the formation tester bodythat can consequently be positioned at any point between the extendedprobe and anchor. It is thus possible of the entire tester body to bepositioned against the wall of the wellbore where the mudcake may havebeen removed while lowering the tool, which drastically increases therisk of being stuck while performing a measurement.

Large rings or standoffs have been used to provide a space or standoffbetween the tool body and the wall of the wellbore, in order to minimizethe risk of sticking. The purpose of these standoffs is to prevent thetool from directly contacting an area of removed mudcake. Document U.S.Pat. No. 5,233,866 discloses a tester wherein a pad provided withmeasurements means on a support plate can be extended simultaneouslywith anchoring means in order to contact the wall of the borehole. Inits extended position, this pad may allow a standoff between the entiretool body and the wall of the borehole.

The drawback of these tools is that the standoffs are not integralportion of the tool body but are bolted, threaded or strapped into thetool body. As a result, they can fall or be torn from the tool bodyduring use in the wellbore. Metal debris falling to the bottom of thewellbore will interfere with the drilling and other developmentoperations of the well. They would consequently need to be removed by acostly and time consuming process. Furthermore, in many cases whileusing this tool, due to the fact that there is no imbalance between theprobe and anchor forces, the tool body can consequently be positioned atany point between the extended probe and anchor. The tool body maytherefore be entirely pressed against the surface of the wellbore,increasing the risk of differential sticking.

It thus remains a need to eliminate the risk of differential stickingwhile performing pretests with a device avoiding any inconvenience oftesters known in the art. It is thus an object of the invention topropose a formation tester for determining the formation pressure of asubsurface formation traversed by a wellbore, said formation testercomprising:

-   -   an elongate tester body;    -   a support plate that is extendible outwardly from the surface of        the formation tester body; said support plate carrying probe        means to establish a passageway between the inside of said        formation tester body and said formation, and a sealing pad        connected to said probe means to isolate said passage way        between the inside of said formation tester body and said        formation;    -   anchoring means to settle said tester body at a level within the        wellbore.

According to the invention, said elongate tester body comprises aneccentric portion wherein said support plate is mounted such that adetermined standoff is maintained between said elongate tester body andthe wall of the wellbore when said tester body is settled at a level inthe wellbore.

Due to the determined standoff, the amount of tool surface area incontact with an area of disturbed or removed mudcake will be drasticallyminimized, which will subsequently minimize the chance of becomingdifferentially stuck to the side of the wellbore while performing apressure measurement. This feature thus enables to perform quicker andsafer pressure measurements (or any other measurement like taking fluidsamples for example) in the wellbore.

In a preferred embodiment for the formation tester of the invention,said tester further comprises probe positioners that are mounted on afirst side of said eccentric portion and extend the support plateoutwardly from the surface of the formation tester body towards the wallof the wellbore. Furthermore, the anchoring means are situated on theside of the tester body opposite to the support plate and there is animbalance between the anchoring force and the force applied by the probepositioners.

Thanks to the imbalance between the probe positioners force and theanchoring means force, one can properly settle the tool inside thewellbore at the measurement level and make sure that the tool is alwayspositioned in the well such that the only contact with the wall of thewellbore will be the eccentric portion surface. This feature will thusenable, even in deviated or horizontal wells to minimize the risk of thetool to remain stuck at the measurement level.

According to a preferred embodiment of the formation tester of theinvention, a hydraulic circuit actuates the probe positioners and theanchoring means, said hydraulic circuit being designed to minimize thetime needed to extend the support plate and settle the tool body.Furthermore, the probe positioners and the anchoring means comprisepistons connected to said hydraulic circuit, the pistons from said probepositioners being of smaller diameter than the diameter of the pistonsfrom said anchoring means.

This feature enables in a very simple way to provide a mechanical forceimbalance between the probe side and its opposite side in order to makesure that the tool is always positioned in such a manner that theeccentric portion of the tool body contacts the wall of the well when apressure measurement is performed.

Advantageously, the eccentric portion of the tester body is an integralpart of the elongate tester body. The fact that the eccentric portion isintegral to the tool body, and is not fastened to said tool body by anyadditional parts enables to maintain a constant standoff between thetool and the borehole wall in any case. Furthermore, this featureprevents this eccentric portion from being modified or lost in thewellbore. This standoff must be significant enough to exceed thethickness of most mudcakes. Typically, the standoff will be at leasthalf of an inch.

It is also proposed to provide a method for performing a formationpressure test of a subsurface formation traversed by a wellbore, saidmethod comprising the following steps:

-   -   lowering an elongate formation tester body inside said wellbore;    -   stopping said formation tester body at a level wherein a        pressure test is to be performed;    -   extending a support plate at said level, outwardly from the        surface of the formation tester body towards the wall of the        wellbore;    -   extending anchoring means to settle said formation tester body        in the wellbore;    -   pressing a sealing pad and probe means carried by said support        plate against the wall of the wellbore to establish a passageway        between the inside of said formation tester body and said        formation and isolate said passageway from the wellbore;    -   performing a formation pressure test,        characterized in that the method further comprises the step of        maintaining a determined standoff between the formation tester        body and the wall of the wellbore, against which said probe and        sealing pad means are pressed, by means of an eccentric portion        of said formation tester body.

Additional objects and advantages of the invention will become apparentto those skilled in the art upon reference to the detailed descriptiontaken in conjunction with the provided figures:

FIG. 1 represents a eccentric view of the formation tester according tothe invention, said view being focused on the eccentric portion of thetool body;

FIG. 2 represents a schematic view of the formation tester according tothe invention, while performing a pressure measurement.

As it can be seen on FIG. 1, the formation tester 10 according to apreferred example of the invention comprises an elongate tool body 1which is lowered in the wellbore via a cable, not shown, and stopped ata depth where a pressure measurement is desired. In a preferredembodiment of the tool of the invention, the tool body is designed to beparticularly light and small, which participates to decrease the risk ofdifferential sticking of the tool and contributes to lower the timeneeded to remove this tool from one place to another.

This elongate tester body comprises an eccentric portion 2 that isintegral with said body, i.e. that cannot be removed or altered duringthe lowering in the wellbore. Typically, this eccentric portion ismachined as one piece with the elongate tool body. It could also be acasted part of the formation tester body or it may also be an externalpart that has been welded to said body. This eccentric portion enablesto create a determined standoff between the wall of the wellbore and theformation tester body, which reduces significantly the risk for saidtool to remain stuck due to the differential pressure between thewellbore and the formation. The standoff depends on the size of theeccentric portion. It must be significant enough to exceed the thicknessof the mudcake that covers the wall of the wellbore and whichalteration, mostly due to the lowering of the tester, causes a risk ofdifferential sticking. Considering the thickness of the mudcake that canbe ½ inch or larger, depending on the permeability of the formation, mudtype, drilling operations, procedure and pressure differential betweenthe inside of the wellbore and the inside of the formation, the standoffresulting from the eccentric portion 2 may be of at least ½ inch.

In an embodiment of the formation tester according to the invention,additional standoffs 11 (see FIG. 2) may be added to the externalsurface of the eccentric portion of the tool body. Furthermore, otherstandoffs can also be added on the tool body apart anywhere else thanaround the eccentric portion. These standoffs will thus be helpful toavoid any sticking of the tool especially on the side opposite theeccentric portion. Any of these standoffs may be of elastomeric ormetallic material and removable from the tool body such that theformation tester according to the invention can also be lowered in wellsof smaller diameter. In an advantageous embodiment of the apparatus ofthe invention, these standoffs are coated with a non-sticking material,ex. Teflon.

A support plate 3 is carried by the external part of the eccentricportion 2. This support plate is extendible outwardly from the surfaceof the formation tester body by mean of probe positioners 4. The probepositioners 4 comprise, as shown as an example in FIG. 1, two pistonsthat are connected to a hydraulic circuit, not shown. Probes means 5 arepositioned in the support plate 3 such that they contact the wall of theborehole when a pressure measurement is performed, as it will beexplained here under with reference to FIG. 2. These probe means createsa passageway between the inside of said formation tester body and saidformation. A sealing pad 6 surrounds said probe means in order toisolate said passageway from the wellbore during a pressure measurement.An elastomeric seal, for example, constitutes the sealing means. In theretracted position, the surfaces of the support plate 3, sealing pad andprobe means are substantially at the same level than the surface of theeccentric portion 2 or lower

Not represented on FIG. 1 but as known in the state of the art, theprobe means 5 are connected to a flowline inside the formation tester.Said flowline is connected to pressure gauges, in order to performpressure measurement on the formation surrounding the borehole.Furthermore, an equalization valve (not shown) enables to equalize thepressure in the flowline to the hydrostatic pressure of the fluid in thewellbore before setting the tool, and after a pressure measurement hasbeen performed. The actuation of this valve enables to remove the toolfrom the wellbore wall before moving to another measurement level. Apressure sensor or gauge is used to continuously measure the hydrostaticpressure of the fluid in the wellbore. In a preferred embodiment of aformation tester according to the invention, the global volume of theflowline is minimized such that time needed to perform the pressuremeasurement is significantly decreased, thus leading to decreasing ofthe differential sticking risk.

Anchoring means 7 are positioned on the other side of the tester body,opposite the eccentric portion 2. For example, this anchoring meanscomprises two pistons that are connected to a hydraulic circuit, notshown. In an advantageous embodiment of a formation tester according tothe invention, the motor that drives the hydraulic circuit is chosen tominimize the time needed to extend and retract said pistons in order tofurther reduce the time needed to perform the pressure measurements andconsequently reduce the risk of differential sticking. A force imbalanceexists between the probe positioners' force, on the eccentric portionside, and the anchoring means force, opposite this side. Due to thisfeature, the position of the formation tester according to the inventionis fully controlled compared to the tester of the state of the art,wherein the position of the tool varies from time to time. The forceimbalance is such that the tester always contacts the wall of thewellbore by the surface of the eccentric portion of the tool body.

Consequently, a determined standoff is always maintained between theformation tester and the wall of the formation, the size of saidstandoff being determined by the size of said eccentric portion. Theforce imbalance might be significant enough to lift the weight of theformation tester when used in horizontal or deviated wells. At least,the force imbalance should be equal to the weight of the tool. In theexample wherein the probe positioners and the anchoring means comprisepistons, this force imbalance may be implemented by providing pistons ofsmaller diameter for said probe positioners than the diameter of thepistons for said anchoring means. Consequently, a larger part of theforce provided by the hydraulic circuit will be transmitted to theanchoring means, thus creating a force imbalance.

Referring now to FIG. 2, representing a wireline formation testingoperation, the formation tester 10 according to the invention is loweredinto a wellbore 8 by a wireline cable 9. While said formation tester islowered in the wellbore, the equalization valve is open, which allowsthe Bowline pressure to be equal to the hydrostatic pressure of thewellbore. When the tool is settled at the measurement level, theequalization valve is closed and the measurement is started. After thepressure measurement is complete, the equalization valve is opened sothat the anchoring means can be retracted and the formation tester canbe moved to a new depth.

The formation tester may then be settled by anchoring the tester inplace with the probe positioners and the anchoring means through thehydraulically actuated pistons. Consequently, at the level where thepressure measurement is desired, the probe positioners extend thesupport plate 3 outwardly from the tester body surface until it reachesthe wall of the wellbore. At that moment, the probe means 5 establishfluid communication with the formation through a passageway. By the sametime the anchoring means is extended from the formation tester until itcontacts the wall of the wellbore opposite the support plate 3. Due tothe force imbalance between the probe positioners and the anchoringmeans, the tool is automatically eccentered in the well, such that itcontacts the wall of the wellbore only on the eccentric portion surface.

When the formation tester according to the invention is settled, thesealing pad is pressed against the wall of the wellbore, around theprobe means, to isolate the interior of the tool from the wellborefluids and the equalization valve is actuated. The point at which a sealis made between the probe means and the formation and at which fluidcommunication is established by the passageway between the inside ofsaid formation tester body and said formation, is referred to as the“tool set” point. As known with conventional formation tester tools inthe state of the art, fluid from the formation is then drawn into theformation tester to create a pressure drop between the flowline and theformation pressure. This volume expansion activity is referred to as a“drawdown” step.

When this drawdown stops, fluid from the formation continues to enterthe probe means through the passageway until, given a sufficient time,the pressure in the flowline is the same as the pressure in theformation. This activity is referred to as a “build-up” step. The finalbuild-up pressure, is usually assumed to be a good approximation to theformation pressure. Data generated by the pressure trace may be used todetermine various formation characteristics. For example, the pressureprofile measured during drawdown and build-up may be used to determineformation mobility that is the ratio of the formation permeability tothe formation fluid viscosity. As already mentioned, the drawdown andbuildup times can be significantly reduced by minimizing the globalvolume of the flowline, thus decreasing the risk of differentialsticking.

After the formation pressure measurement cycle has been completed, theformation tester may be disengaged and repositioned at a different depthand the formation pressure test cycle repeated as desired. Actually,when disengagement is required, the equalization valve is opened toequalize the pressure between the flowline inside the tool and thehydrostatic pressure of the wellbore. Then, both probe positioners andanchoring means are actuated in the reverse way and enter in the insideof the tester body. The probe means are thus disengaged from thewellbore wall, the pressure in flowline increases rapidly as itequilibrates with the wellbore pressure.

Thanks to the eccentric portion 2 of the tester body, the risk ofremaining stuck against the wall of the wellbore due to differentialpressure is significantly lowered. Furthermore, the reduction of thetool area in contact with the wellbore and the precised positioning ofthe tool by mean of the force imbalance between the positioners forceand the anchoring force is of significant help to overcome said risk.

1. A formation tester for determining the formation pressure of asubsurface formation traversed by a wellbore, said formation testercomprising: an elongate tester body (1); a support plate (3) that isextendible outwardly from the surface of the formation tester body, saidsupport plate carrying probe means (5) to establish a passageway betweenthe inside of said formation tester body and said formation, and asealing pad (6) connected to said probe means to isolate said passageway between the inside of said formation tester body and said formation;anchoring means (7) to settle said tester body at a level within thewellbore; wherein said elongate tester body comprises an eccentricportion (2) wherein said support plate is mounted such that a determinedstandoff is maintained between said elongate tester body (1) and thewall (8) of the wellbore when said tester body is settled at a level inthe wellbore.
 2. A formation tester according to claim 1, furthercomprising probe positioners (4) that are mounted on a first side ofsaid eccentric portion (2) and extend the support plate (3) outwardlyfrom the surface of the formation tester body towards the wall of thewellbore.
 3. A formation tester according to claim 2, wherein theanchoring means (7) are situated on the side of the tester body oppositeto the support plate and wherein there is an imbalance between theanchoring force and the force applied by the probe positioners (4).
 4. Aformation tester according to claim 3, wherein the force imbalance issuch that the anchoring means press the surface of the eccentric portion(2) of the tool body against the wall of the borehole.
 5. A formationtester according to claim 3, wherein the force imbalance is equal orhigher than the weight of said formation tester.
 6. A formation testerbody according to claim 2, wherein a hydraulic circuit actuates bothprobe positioners and anchoring means, said hydraulic circuit beingdesigned to minimize the time needed to extend the support plate andsettle the tool body.
 7. A formation tester body according to claim 1,wherein the probe positioners and the anchoring means comprise pistonsconnected to said hydraulic circuit, the pistons from said probepositioners being of smaller diameter than the diameter of the pistonsfrom said anchoring means.
 8. A formation tester according to claim 1,wherein the eccentric portion of the tester body is an integral part ofthe elongate tester body.
 9. A formation tester according to claim 1,wherein the degree of standoff of the eccentric portion of the testerbody is such that the resulting standoff provided between the elongatetester body and the wall of the wellbore is at least one half of aninch.
 10. A formation tester according to claim 1, wherein the probemeans are connected to a pressure sensor that communicates with thepassageway between the inside of the formation tester body and theformation.
 11. A formation tester body according to claim 1, whereinadditional standoff means (11) are secured on said elongate tester body,said additional standoff means being removable.
 12. A formation testeraccording to claim 11, wherein said additional standoff means are coatedwith a non-sticking material.
 13. A method for performing a formationpressure test of a subsurface formation traversed by a wellbore, saidmethod comprising the following steps: lowering an elongate formationtester body inside said wellbore; stopping said formation tester body ata level wherein a pressure test is to be performed; extending a supportplate at said level outwardly from the surface of the formation testerbody towards the wall of the wellbore; extending anchoring means tosettle said formation tester body in the wellbore; pressing a sealingpad and probe means carried by said support plate against the wall ofthe wellbore to establish a passageway between the inside of saidformation tester body and said formation and isolate said passagewayfrom the wellbore; performing a formation pressure test, wherein themethod further comprises the step of maintaining a determined standoffbetween the formation tester body and the wall of the wellbore againstwhich said probe means and sealing pad are pressed by means of aneccentric portion of said formation tester body.
 14. A method accordingto claim 12, wherein there is an imbalance between the force thatpresses the probe means against the wall of the wellbore and the forcethat settles the tester body at the level wherein the pressure test isto be performed.
 15. A method according to claim 13, wherein the forceimbalance is equal or higher than the weight of the formation testerbody.
 16. A method according to claim 14, wherein a hydraulic circuitactuates both probe means and anchoring means, said force imbalance beencreated by this hydraulic circuit.
 17. A method according to claim 16,wherein the hydraulic circuit is designed to minimize the time needed toextend and retract the support plate and the anchoring means.
 18. Amethod according to claim 13, wherein said probe means are connectedwith a flowline inside said formation tester, the volume of saidflowline been chosen to minimize the time needed to perform theformation pressure test.